Transport Minister Khaw Boon Wan, 67, will retire from politics and not contest the general election, ending a political career spanning nearly two decades.

Mr Khaw, who is also Coordinating Minister for Infrastructure, entered politics in 2001 and is known for having tackled thorny issues across various ministries.

In a valedictory letter to the veteran politician yesterday, Prime Minister Lee Hsien Loong noted that Mr Khaw has helped Singapore overcome challenges spanning healthcare, housing and transport.

"In your quiet, unassuming way, you have made a huge and lasting contribution, and strengthened Singaporeans' faith that this Government can and will solve their problems and improve their lives," wrote PM Lee.

As Senior Minister of State for Health during the severe acute respiratory syndrome (Sars) outbreak in 2003, Mr Khaw was on the front line, checking on arrangements and encouraging hospital staff, he said.

He also instituted new processes that were applied when Covid-19 struck this year, said PM Lee.

In 2010, Mr Khaw had a heart bypass when he was Health Minister. He paid $8 out-of-pocket for the surgery, thanks to MediShield, Medisave and his private insurance, and used the incident to highlight the importance of health insurance and how government policies kept healthcare costs low for Singaporeans.

After the 2011 General Election, he helmed the National Development Ministry. Housing affordability was a hot-button issue that year. Mr Khaw "ramped up the HDB building programme", cutting waiting times and enabling young Singaporeans to own their first homes earlier through various policies, said PM Lee.

One of the biggest challenges Mr Khaw took on was to improve the MRT system, added PM Lee.

In 2015, when he was appointed Transport Minister, Singapore's MRT trains travelled an average of 133,000km between delays of more than five minutes - known as mean km between failure (MKBF).

Mr Khaw set a target of one million MKBF and many thought it was "unrealistic", but this target has since been surpassed, said PM Lee. In the first quarter of this year, the MRT system hit 1.4 million MKBF.

"This dramatic achievement is the product of a huge amount of hard work, your encouragement for the regulator and rail operators to work together as one team, your strengthening of rail engineering capabilities, and your emphasis on the crucial but often unnoticed work of continuous system maintenance and timely asset renewal," said PM Lee.

The Prime Minister called Mr Khaw, who has been in his Cabinet since he became PM in 2004, "one of my most reliable lieutenants" and thanked him for his personal advice and friendship".

He pointed out that Mr Khaw had also made crucial contributions to the People's Action Party. After the party saw its vote share fall to 60.1 per cent in the 2011 election, Mr Khaw led introspective efforts to identify where it had fallen short and "set the party on a fresh course". It contributed to the party's decisive win in the next general election, he said.

"For the younger ministers, you have been a role model and a source of sage advice. They look to you to learn not only how to solve difficult problems, but also your seemingly effortless way of explaining nettlesome issues to Singaporeans."

Mr Khaw's political juniors took to Facebook to pay tribute to him.

"He has been my teacher, my mentor, and helped shaped my career. He has been such a great influence, I would describe him as a Titan Minister," wrote Education Minister Ong Ye Kung, who will succeed Mr Khaw as the PAP team's anchor in Sembawang GRC.

In a Facebook post yesterday, Mr Khaw thanked his constituents, civil servants, colleagues in Parliament and the Cabinet. "After 42 years of public service. It is a week of high emotion for me. Gratitude fills me to the brim!" he wrote.

The former Colombo Plan scholar had been in public service since he graduated, and at one point served as principal private secretary to Mr Goh Chok Tong when he was prime minister. Mr Khaw also worked as permanent secretary at the Ministry of Trade and Industry before entering politics in 2001.

"Throughout this journey, my wife stands with me, as my strongest anchor. Our children and grandchildren remind me of the joys of family life, and the larger purpose of nation-building: It is to secure future generations of Singaporeans an even better life," he wrote.

Ms Poh Li San, Changi Airport Group's vice-president for Terminal 5 Planning, is expected to replace Mr Khaw on the PAP slate for Sembawang GRC. She was officially introduced by the party on Thursday.

PM Lee thanked Mr Khaw for his service and said he was glad he would continue to advise newer ministers after retirement. In a Facebook post, he said: "On behalf of all Singaporeans, and especially residents in his Sembawang constituency, I thank Boon Wan for all that he has done, and wish him a happy, healthy and well-deserved rest."